Soh Kushida

Self-assembled conjugated polymer spheres as fluorescent microresonators

Confinement of light inside an active medium cavity can amplify emission. Whispering gallery mode (WGM) is one of mechanisms that amplifies light effectively by confining it inside high-refractive-index microstructures, where light propagates along the circumference of a sphere via total internal reflection. Here we show that isolated single microspheres of 2–10 μm diameter, formed from self-assembly of π-conjugated alternating copolymers, display WGM photoemission induced by laser pumping. The wavelengths of the emission peaks depend sensitively on the sphere size, position of the excitation spot and refractive index of each polymer. The Q-factor increases with increasing sphere diameter and displays a linear correlation with the reciprocal radius, indicating that the small curvature increases the efficacy of the total internal reflection. WGM photoemission from π-conjugated polymer microspheres is unprecedented and may be of high technological impact since the microspheres fulfill the role of fluorophores, high-refractive-index media and resonators simultaneously, in addition to their simple fabrication process.

Conjugated Polymer Blend Microspheres for Efficient, Long-Range Light Energy Transfer

Highly luminescent π-conjugated polymeric microspheres were fabricated through self-assembly of energy-donating and energy-accepting polymers and their blends. To avoid macroscopic phase separation, the nucleation time and growth rate of each polymer in the solution were properly adjusted. Photoluminescence (PL) studies showed that efficient donor-to-acceptor energy transfer takes place inside the microspheres, revealing that two polymers are well-blended in the microspheres. Focused laser irradiation of a single microsphere excites whispering gallery modes (WGMs), where PL generated inside the sphere is confined and resonates. The wavelengths of the PL lines are finely tuned by changing the blending ratio, accompanying the systematic yellow-to-red color change. Furthermore, when several microspheres are coupled linearly, the confined PL propagates the microspheres through the contact point, and a cascade-like process converts the PL color while maintaining the WGM characteristics. The self-assembly strategy for the formation of polymeric nano- to microstructures with highly miscible polymer blends will be advantageous for optoelectronic and photonic device applications.

Poly(aryleneethynylene) Tongue That Identifies Nonsteroidal Anti-Inflammatory Drugs in Water: A Test Case for Combating Counterfeit Drugs

We report a sensor array composed of a highly fluorescent positively charged poly(para-phenyleneethynylene) P1 and its complex C with a negatively charged pyridine-containing poly(para-aryleneethynylene) P2 (quencher) at pH 10 and pH 13; a sensor field composed of four elements, P1 (pH 10), P1 (pH 13), C (pH 10), and C (pH 13), results. The elements of this small sensor field experience either fluorescence turn on or fluorescence quenching upon exposure toward nonsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin, ibuprofen, diclofenac, or naproxen. The combined responses of the sensor field are analyzed by linear discriminant analysis (LDA). All of the NSAIDs were identified and discriminated, and the sensing mechanism, hydrophobic versus electrostatic, was discussed.

Optically induced mode splitting in self-assembled, high quality-factor conjugated polymer microcavities.

We investigate the whispering gallery modes (WGMs) of self-assembled single microspheres. They consist of a recently developed highly fluorescent π-conjugated copolymer and exhibit excellent optical properties with Q-factors up to 104. Under continuous laser irradiation, we observe a splitting of the highly degenerate spherical WGMs into a multiplet of lines. Comparison with the calculated spectral response of a weakly distorted sphere shows that the optical excitation induces a change of the optical path length in the microcavity so that it resembles a prolate spheroid. The separation of the lines is given by the ellipticity and the azimuthal mode number. Measurements in various gaseous environments suggest that the distortion is caused by light induced oxidation of the polymer. Our findings show that photooxidation can be a beneficial mechanism for in-situ tuning of optically active polymer structures.

Tetramethylbithiophene in π-conjugated alternating copolymers as an effective structural component for the formation of spherical assemblies

π-Conjugated alternating copolymers containing a tetramethylbithiophene unit show a strong tendency to form well-defined, sub- to several-micrometer-sized spheres. The twisted bithiophene unit inhibits interchain stacking and anisotropic crystal growth of these copolymers, leading to the formation of structurally isotropic spheres by means of a slow diffusion of a nonsolvent into a solution of the copolymers. These micrometer-sized spheres display extremely long photocarrier lifetimes (∼10−3 s) in comparison with cast films from the solutions of the polymers and those of the irregular aggregates (<10−6 s).

Self-assembled polycarbazole microspheres as single-component, white-colour resonant photoemitters

Self-assembled polycarbazole (PCz) microspheres exhibit whispering gallery mode photoluminescence (PL), where resonant PL lines appear at the whole visible spectral range. The ultra-wide-range PL results from a partial oxidation of PCz upon strong photoexcitation. The single component micro-photoemitters, preparable using a simple and inexpensive process, are applicable as full-colour, narrow-width light emission sources.

From Linear to Foldamer and Assembly: Hierarchical Transformation of a Coplanar Conjugated Polymer into a Microsphere

Despite the coplanar structure, a conjugated alternating copolymer forms amorphous, well-defined microspheres without π-stacked crystalline domains. Here, we gain insights into the mechanism of how the coplanar conjugated polymer forms amorphous microspheres by means of spectroscopic studies on the assembly/disassembly processes. The difference of the spectral profiles of photoabsorption and photoluminescence with varying solvent/nonsolvent composition clarifies that stepwise assembly takes place through the microsphere formation; [1] intrapolymer linear-to-folding transformation upon diffusion of polar nonsolvent and [2] interpolymer assembly of the foldamers upon further addition of the nonsolvent to form microspheres. As shown in various biopolymers such as proteins and DNA, such stepwise folding and assembly behaviors of conjugated polymers from primary to secondary and tertiary structure open a new way to create transformable functional materials.

FRET-mediated near infrared whispering gallery modes: studies on the relevance of intracavity energy transfer with Q-factors

Near infrared (NIR) optical microsphere resonators are prepared by coassembly of energy-donating and accepting conjugated polymers. In the microspheres, fluorescence resonance energy transfer occurs, leading to sharp and periodic photoluminescence from whispering gallery modes in the NIR region with Q-factors as high as 600.

Whispering gallery mode photoemission from self-assembled poly-para-phenylenevinylene microspheres

Poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMOPPV) self-assembles to form well-defined spheres with several micrometers in diameter upon addition of a methanol vapor into a chloroform solution of MDMOPPV. The single sphere of MDMOPPV with 5.7 µm diameter exhibits whispering gallery mode (WGM) photoemission upon excitation with focused laser beam. The periodic emission lines are characterized by transverse electric and magnetic WGMs, and Q-factor reaches ∼345 at the highest.